We are already familiar with a variety of devices capable of carrying out the monitoring of polarization mode dispersion, PMD, in photonic transmission systems. In a number of patent applications different techniques are described for the measuring and monitoring of PMD in systems of this type. These known techniques, however, may prove inadequate or, at least, open to improvement, insofar as spectral resolution and measuring speed are concerned, which may be crucial in the field of application of the present invention.
Patent application publ. no. US-A-2004/008991 describes a method for measuring PMD by means of heterodyne detection (modulus and phase) of the signal produced by the optical mixing of the signal present in a photonic transmission system with an optical carrier generated by a continuous wave laser, which has a variable oscillation frequency. In this way, by scanning the whole spectrum of the signal, it is possible to calculate the value of the system PMD. However, the scan performed by the laser is a complex inherently slow process. PMD is a random process which may have a highly variable dynamic, which means that it may be important to increase the monitoring speed. The polarization state and PMD are evaluated on the basis of amplitude and frequency detected by heterodyne means, which is a method intrinsically different from the one considered in the present patent. The system described in US-A-2004/008991 requires a mixing process, a continuous wave laser and a heterodyne detection mechanism, which entails a fairly complex system.
Patent application publ. No. JP-A-2004-138615 also describes a system for measuring PMD in a photonic link, by means of the coherent analysis (including measuring the amplitude and phase) of the spectrum of a specific signal divided into bands, transmitted by way of said link. The method proposed, however, is based on specific signals, which prevents the link from being in operation at the same time and requires a heterodyne polarimeter, which makes a sweep in frequency with a local oscillator variable for each band. This method suffers from the same limitations as the application described in the previous paragraph with regard to complexity and speed of operation.
Furthermore, patent application publ. no. US-A-2004/151416 describes a method for measuring PMD based on the division into sub-bands of the frequency range encompassed by the signal transmitted by the photonic transmission system. A reference signal is calculated on each one of these bands. On this reference signal a mix is done with a signal from a laser source. The value of the PMD of the original system signal is calculated by collating the information from all the sub-bands. This method is substantially complex and limits spectral resolution to the number of sub-bands implemented in the device.
Furthermore, patent application publ. no. EP-A-1494373 describes a PMD analyser based on the distortion of the optical signal transmitted. This distortion is evaluated on the basis of the direct (DC) and alternating components (AC) of the signal, which indicate the distortion introduced by PMD. This approach requires a set of filters located precisely at certain frequencies, its configuration being dependent on the system signalling rate.
We have considered that it would be beneficial to establish a method and device which resolves, at least partly, some of the problems or drawbacks presented by the known systems.